Method of epitaxially growing single crystal films of metal oxides

ABSTRACT

A method of epitaxially growing single crystal ferrite films by vapor transport comprises forming gaseous metal halides from metal oxides and decomposing these metal halides to form a single crystal film of a desired ferrite composition upon a single crystal substrate.

United States Patent Joseph John Hanak Trenton, NJ. 748,757

July 30, 1968 Nov. 2, 197 1 RCA Corporation Inventor Appl. No. FiledPatented Assignee METHOD OF EPITAXIALLY GROWING SINGLE CRYSTAL FILMS OFMETAL OXIDES References Cited UNITED STATES PATENTS Moest Chu et a1. Meeet al....

Mee Pulliam Burd et a1.

Primary Examiner-Herbert T. Carter Attorneys-Glenn H. Bruestle and JoelF. Spivak 23/50X 117/106 ll7/169X 117/169X 1l7/106X 117/106X ABSTRACT: Amethod of epitaxially growing single crystal ferrite films by vaportransport comprises forming gaseous metal halides from metal oxides anddecomposing these metal halides to form a single crystal film of adesired ferrite composition upon a single crystal substrate.

PATENTEUuuv 2 I97! 3.617. 381

1; a 4n! fig lllli/ INVENTOI ATTOIIIY METHOD OF EPITAXIALLY GROWINGSINGLE CRYSTAL FILMS OF METAL'OXIDES BACKGROUND OF THE INVENTION Thisinvention relates to the synthesis of single crystal thin films of metaloxides and particularly to an improved process for growing singlecrystal films of ferrimagnetic oxides.

Ferrimagnetic films are useful, for example, as memory elements, asmicrowave devices such aslimitors and delay lines, as magnetic recordinghead surfaces, and in magneto-optic devices. The magnetic properties offerrimagnetic materials, such as ferrites, depend largely upon thechemical composition, the valence of the constituentelements and;themicro structure of the material. In order to obtain high quality ferritefilms, many variables must be carefully controlled these variablesinclude temperature, time and-atmosphere.

Recently, single crystal films of magnetic oxides have been prepared bythe chemical vapor transport of the constituent metal halides in acarrier gas. In accordance with these techniques, each metalhalideisseparately heated toatemperature above its melting points and acarrier. gas is passed thereover so as to carry vapors of themetalhalide. The. gas flow over each halide mustbe separately adjustedinorder to obtain the desired proportionsof the various metals whichreact to form the magnetic film. The gaseous reactants in these priorart techniques are brought together to form a reaction mixture, whichmixture also includes water vapor. The introduction of the water vaporinto the reaction mixture'must also be separately controlled. .Inaddition tothe aforementioned controls over the various constituents ofthe reaction mixture, the temperature of the reaction chamber must alsobe separately controlled in order to cause the reactants to form thedesired product. These many variables which must be controlled accordingto the prior art techniques make these techniques difficult to perform,difficult to control, and difficult to duplicate.

Other prior art methods for producing ferrite films involve vacuumevaporation, sputtering, and the like. These methods either result insingle crystal films having poor magnetic characteristics or inpolycrystalline films.

Thepresent invention discloses a method of producing single crystal thinfilms, which method is significantly easier to control than the priorart procedures, and resultsi'n magnetic sin glecrystal films having goodmagnetic properties.

SUMMARY or THEINVENTION A vapor transport method for the deposition ofsingle crystal metal oxide films comprises the steps of convertingconstituent metal oxides to gaseous products and then converting saidgaseous products to single crystal metal oxide layers by epitaxiallydepositing said metal oxides on a single crystal substrate.

BRIEF DESCRIPTION OF THE DRAWINGS H6. 1 is an elevational partiallysectional view of an apparatus useful in carrying out one embodiment ,ofthe novel method.

FIG. 2 is an elevational, partially sectional view of an apparatus usedto carry out another embodiment of the novel method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel method is applicableto the production of single crystal films of metal oxides generally, andis particularly useful for the production of single crystal magneticthin films.

A. Close-Space Vapor Transport C lose-space vapor transport is similarto closed vapor transport systems in that the chemical reactionsinvolved are essentially the same. However, the close-space systemdiffers from the prior art closed systems in that 1. it is done in anopen tube under a very low flow rate of reactant hydrogen halide gasesas compared to the completely sealed tube equilibrium conditions of theprior art systems,

2. single crystal'films are grown by epitaxially-depositing the reactionproduct on a single crystalsubstrate as compared to the growth ofcrystallites'at one end of the sealed reaction tube as occurs in theprior art systems.

The, novel close-space transport method employs a starting material of ametal oxide, for example, a sintered ferrite of essentially the samecomposition as the desired film to be produced. This starting materialis preferably held in acontainer such as platinum, which is inert to theinitial reactants as well as to the intermediate metal halide vaporsproduced during the reaction. According to the novel method, theflowrate of the reactant hydrogen halide gas is very low, preferably in theorderof l to 5 milliliters per minute, and typically less than about 10milliliters per minute, providing near equilibrium conditions. Thesubstrate upon whichthe ferrite film is formed is placed in closeproximity to the ferrite starting material. Typically, the distancebetween the starting material and the substrate is inthe order of l to 5millimeters; although distances of up to about 10 millimeters arepractical and distances of from about 1 to 3 millimeters are preferred.This distance is important since at the operating gas pressure the gasismoved essentially by diffusion andthe process is limited by thediffusion of the gases, thereby making the rate of depositionapproximately inversely proportionalto the square of thedistance betweenthe reactant metaloxideand the substrate. There is a temperaturegradient between the reactant and the substrate in the close-spacetechnique. The temperature. gradient is such that the temperature at thesurface of the substrate is less than that of the reactant. Typically,the temperature of the substrate is about 20to 30 degrees centigradecooler than that of'the reactant metal oxide.

An apparatus 10 useful for the operation of this novel closespacevaportransport method for producing thin metaLoxide films is shown inFIG. 1. The apparatus 10 comprisesia 're- .sistance furnace 11 which hasa flat or planar inner bottom surface 12. Under the surface 12 is aheating element 13 extending below and along'the length of the innerbottom surface 12 of;the furnace 11. A reaction tube '14 of a materialsuch as quartz which is relatively inert to the gases present thereinduring the reaction and which can be heated to the temperaturesrequired, is provided. The reaction tube 14 which is placed in thefurnace 11 as shown, is closed at one end 15 and has a flat bottomsurface 16 which rests on the fiat bottom surface 12 of the furnace '11.The open end of the tube 14-is provided with a gasinlet 17 and a gasexhaust 18 which may be connected to the tube 14 by means of a balljoint assembly 19.

In operation, the reaction tube 14 is positioned in the furnace 11 andthe furnace is loaded with two inert containers 20 and 21, whichcontainers 20 and 21 contain powdered ferrite 22 of essentially the samecomposition as the final film to be deposited. The containers 20 and 21,which can be made of platinum, are preferably rectangular in crosssection so as to fit flatly on the bottom of the tube 14. The containers20 and 21 are placed in the tube 14 one behind the other. The container20 farthest from the gas inlet 17 has a single crystal substrate 23, offor example magnesium oxide, placed directly over an opening 24 in itstop lid or surface. The distance between the substrate 23 and the levelof the ferrite 22 within the container 20 is preferably between about Ito 3 millimeters. The container 21 nearer the gas inlet is shown ashaving a plurality of openings 25 in its top portion so as to providefor the flow of gas in and out of the container 21.

After placing the containers 20 and 21 in the furnace 11 as described,it is preferable to place a quartz wool plug 26 in the tube 14 as shown.The plug 26 acts both as a filter for incoming gas and a trap andcondensation area for outgoing gases.

The ball joint assembly 19 which is provided with the gas inlet 17 andthe gas outlet 18 is then connected onto the tube 14 and the furnace 11is brought to operating temperature under an inert gas atmosphere. Theoperating temperatures typically range from about 850 to 1 150 C. andare preferably between 950 to 1100 C. The inert atmosphere is provided,for example, by allowing helium gas to flow into the tube 14. Since thesintered ferrite sources 22 are nearer the heating element 13, atemperature gradient exists whereby the substrate 23 is maintained at atemperature of from about 20 to 30 C. below that of the ferrite 22within the container 20. When the furnace 11 reaches the desiredoperating temperature, as determined by the particular ferrite and thedesired deposition rate, the gas admitted to the tube is altered toinclude either hydrogen chloride or hydrogen bromide of possibly amixture thereof along with an inert carrier gas. Typically, the flowrate of the gas mixture is about 3 to 6 milliliters per minute withabout onehalf the gas being the hydrogen halide. As the gases reach theferrite 22, the following reaction occurs: M 20 (s)+bh8HX(g)=MX,(g)+2FeX -,(g)+4H 0.(g), wherein M represents the metal ion in the ferrite 22other than iron and X represents the particular halide used in thereaction. The gaseous products produced, which are indicated on theright hand side of the above equilibrium equation, diffuse from thesurface of the ferrite 22 in container 20 to the cooler surface of thesingle crystal substrate 23 where they then recombine to form a singlecrystal ferrite film 28 on the surface of the substrate 23. Thethickness of the film 28 depends upon the rated deposition which isgenerally in the range of about 2 to 8 microns per hour, and the totaldeposition time.

Generally, the rate of formation and the rate of diffusion of the ferrichalide and the water vapor are greater than that of the other metalhalide vapors formed during the reaction. This results in a smallportion of these materials escaping from the region of the container 20and the substrate 23 thereon, and consequently causes a deviation fromequilibrium conditions. This in turn can cause a variation in thecomposition of the ferrite film. which variation would be greater withtime. Such a variation is effectively and substantially eliminated bythe presence of the ferrite 22 in the container 21, which acts as amakeup device for maintaining relatively constant equilibrium conditionsaround the region of the substrate 23 by being a source for thereplenishment of the lost gases. In addition, the very slow flowratesemployed and the quartz wool plug 26 also minimizes this effect.

lt should be noted that the novel method may be used to produce singlecrystal films of ferrites doped with or containing several, metal ionsby, for example, either adding such metals to the ferrite powderstarting material in the form of their oxides or other forms which willultimately react with the hydrogen halide to produce a volatile metalhalide at the reac tion temperature or by starting with a ferrite powdercontaining these metals therein or by starting with a mixture of oxidesrather than ferrite and including oxides of these dopant metals.

Example 1 This example describes the preparation of single crystal filmsof magnesium manganese ferrite. The platinum containers 20 and 21 are 4inches long, 1 inch wide and onefourth inch high and they are filled towithin about 2 millimeters from the top with powdered magnesiummanganese ferrite comprised of 20, 30 and 60 mole percent of MgO, MnOand Fe o respectively. The temperature of the surface 12 is brought toand held at about 1050 C. A flow ofa mixture of hydrogen bromide gas atabout 1 milliliter per minute and of helium at about milliliters perminute is passed into the tube 14 through the gas inlet 17. Thesubstrates 23 are four single crystal M g0 wafers %X-'%X1/32 incheseach. The temperature of the substrates is approximately 1020 C. Thedesired ferrite film deposits on the surface of these substrates.

The total deposition time of 72 hours yields a film which is about 0.015inch thick on eachof the substrates. The composition of the depositproduced was analyzed to be about 2 percent lower in magnesium and about2 percent higher in iron than the starting material.

Example 2 Synthesis of manganese ferrite. The conditions and the mannerof carrying out the deposition are similar to example 1, except that thetemperature at the surface 12 is brought to 900 C., and the flow of HBris 2 milliliters per minute and that of He 3 milliliters per minute.Correspondingly, the source material is powdered manganese ferrite. ln a50-hour period a 0.012 inch thick deposit is obtained on single crystalsubstrate of manganese zinc ferrite.

Example 3 Synthesis of lithium ferrite. The same procedure is followedas set forth in example 1 except that the surface 12 is held at 1,000 O,and the flow of HBr and helium are both 2 milliliters per minute. Thesubstrate of this example is single crystal sapphire and the source ispowdered lithium ferrite. Epitaxial films 0.003 inch thick are obtainedin 24 hours.

Example 4 Synthesis of nickel ferrite and cobalt ferrite. The sameprocedure as set forth in example 1 is employed except as follows. Thestarting material is powdered nickel ferrite or cobalt ferriterespectively. The gas mixture is a 50 percent mixture of HBr in He witha total rate of flow of 5 milliliters per minute and the temperature atsurface 12 is set at 875 C. Single crystal films, 0.008 inch thick, ofnickel ferrite and cobalt ferrite respectively are obtained in 30 hours.With a mixture of 50 percent HCl in He similar results are obtained whena ferrite temperature of 925 C. is employed.

B. Isothermal Transport In the isothermal transport method, the ferriteis again decomposed and converted to gaseous products according to thereaction described above with relation to the close-space transportmethod. However, according to this method, the temperature of thesubstrate and the ferrite sources are preferably essentially the same(hence, isothermal) such that epitaxial film growth of the ferrite doesnot occur due to a difference in temperature between the substrate andthe ferrite starting material. According to the isothermal transportmethod, the deposition of the single crystal ferrite thin film upon thesingle crystal substrate is caused to occur by injecting water vaporinto the equilibrium mixture of the gases so as to shift the equilibriumand to drive the aforementioned reaction toward the formation of theferrite.

It has been found that although it is possible to obtain single crystalferrite in this manner, there is a gradual change in the depositedferrite composition as the reaction proceeds. This change is believed tobe caused by an unequal depletion of the difierent metals and/or ofoxygen in the ferrite. For this reason, as well as for greaterflexibility in control of the film composition, instead of using aferrite as a source material. separate multiple sources of individualmetal oxides are preferred. For example, in the deposition of manganeseferrite, MnFe O the two source oxides are MnO and Fe O It has also beenfound that during the halogenation of the ferric oxide, especially whenhydrogen bromide is used as the reaction gas, a small quantity of theferric ion is reduced to the ferrous state. This reaction is undesirableand can be prevented by including a small amount of oxygen gas to bemixed with the hydrogen halide gas which passes over the iron oxide.Typically, the volume ratio of oxygen to hydrogen halide is in the rangeof about 1:10 to 1:100 and, preferably. is about 1:20.

An apparatus useful for carrying out the isothermal transport method forthe deposition of epitaxial single crystal thin ferrite films is shownin FIG. 2. The apparatus 30 is comprised of a circular tube furnace 31through which extends a quartz reaction tube 32. The furnace is providedwith heating elements 33 which uniformly heats the reaction tube so asto create an essentially isothermal condition within the central portionof the reaction tube. The reaction tube is open at both ends to allowfor the insertion of the reactants and the'substrate 34. The reactiontube 32 as shown, is provided with an inert gas inlet 35 and two gasoutlets 36 and 37 through which unused gases may be exhausted from thereaction tube. The substrate 34 (or substrates) is contained on asubstrate holder 38 which enters the reaction tube 32 through one endthereof and is held therein by means of a first ball joint assembly 39.The substrate extends into a portion of the reaction tube hereinaftercalled the deposition zone. This zone is preferably of uniformtemperature. The metal oxide reactants 41 and 42 which act as sourcesfor the metals which are part of the final ferrite film, are provided bymeans of oxide containers 43 and 44 which extend into the depositionzone of the reaction tube 32. The oxide container tubes 43 and 44 aresecured by means of a second ball joint assembly 45 which connects tothe open end of the reaction tube 32 opposite that of the substrateholder 38. The apparatus, as shown, indicates the container tubes 43 and44 as open glasses tubes each of which are widened in the region inwhich the oxide is held and each of which is provided with an opening 46and 47 for the entry and exhaustion of gases therethrough. Only two suchcontainer tubes are shown, however, one may provide a separate containerfor each metal ion to be included in the ferrite composition. A secondgas inlet 48 is provided through which the water vapor is admitted tothe reaction tube 32. The water vapor is preferably admitted directlyinto the deposition region of the reaction tube 32.

In operation, the metal oxide containers 43 and 44 are loaded with therespective metal oxides 41 and 42 and inserted into the reaction tube32, as shown. The substrate 34 or substrates are placed upon thesubstrate holder 38 and similarly inserted into the reaction tube 32. Aninert carrier gas is then admitted into the furnace, preferablysimultaneously through all of the gas inlets so as to purge the furnace31 of normal atmosphere. While being purged, the furnace 31 is broughtto operating temperature which is, typically, between about 900 and Il50C. When the desired temperature is achieved, a mixture of HCl or HBrwith an inert carrier gas such as helium, is passed into the containertubes 43 and 44 through inlets 46 and 47 and over the metal oxidestherein. In addition to the above gases, oxygen is included in the gasmixture which passes into the container having iron oxide therein. Theunused gases and the gaseous reaction products of the reaction betweenthe hydrogen halide and the metal oxide in each tube, passes out of thecontainer tubes 43 and 44 and over the substrate 34 in the depositionzone of the reaction tube 32. It is preferable to allow this passage ofgas to take place for a period of at least about minutes to insure thatthe gas mixture in the region of the substrate is an equilibrium gasmixture. After such time has elapsed, water vapor mixed with an inertcarrier gas is caused to enter the deposition zone of the reaction tube32. The water vapor causes a shift in the equilibrium toward theformation of ferrite. This shift in equilibrium results in the epitaxialdeposition of thin ferrite single crystal films upon the substrate.

This technique is extremely flexible in that one can easily achieve acontrol of the composition of the ferrite film. That is, one can easilyadd a number of metal ions to the film by simply providing additionalmetal oxide containers containing such ions or by mixing metal oxidescontaining such ions with other metal oxides the ions of which areincluded in the film. Control of the composition can also be achieved inthat the relative ratio of the metals present in the ferrite can easilybe controlled. For example, the composition of manganese ferrite M nFe Oexpressed as the ratio of iron to manganese in the ferrite can becontrolled between 1.0 and 5.5 by adjusting the relative ratios of thehydrogen halide flow over the respective metal oxide sources.

Example 5 Manganese ferrite (MnFe O is prepared in accordance with thegeneral procedure given above and under the following specificconditions:

Isothermal reaction temperature equals 1,000 C.

Gas flow over MnO consists of 58.5 ml./min. HBr and245 ml./min. He.

The gas mixture over Fe O consists of 95.5 ml./min. HBr, 245 ml./min. Heand 5 ml./min. 0

After the above gas mixtures were passed over the respective oxides for10 minutes, a gas mixture consisting of 4,220 ml./min. of He and 6.2grams/hr. of water vapor was passed into the deposition zone of thereaction tube 32 through the water vapor inlet 47. In addition to theabove gases, He gas, at a rate of 4,220 ml./min. entered the reactiontube through the inlet 35 and 50 mL/min. of He gas was passed into thereaction tube through inlet 37. Under these conditions, the ferritecomposition, as given by the ratio of FezMn, was typically in the rangeof 2.0i0.05:l and the rate of deposition was about 0.0008 inches perhour.

Example 6 In this example singly crystal films of nickel ferrite aresynthesized in accordance with the general procedure set forth above.The starting material in this example is MO and Fe O The furnacetemperature is approximately 950 C. Hydrogen chloride is used in thefollowing amounts to decompose the starting oxides:

Flow ofHCl over NiO 60 ml./min. Flow of HCI over F6303 mL/min.

Example 7 In this example single crystal films of magnesium ferrite aresynthesized in accordance with the general procedure set forth above.The starting oxides used are magnesium oxide and ferric oxide. Thefurnace temperature of 1050' C. is higher than for most other ferritesbecause of the low volatility of magnesium bromide formed during thereaction. Gas flow conditions were similar to those given in example 5,except that the flow of hydrogen bromide over the magnesium oxide wastwo to four times higher than that given for the manganous oxide inexample 5. The reason for this is that magnesium oxide is attacked to alesser degree by hydrogen bromide than manganous oxide.

As indicated, the novel techniques described above have been used toyield single crystal ferrite films of many ferrites includingmagnesium-manganese ferrite, magnesium ferrite, lithium ferrite, nickelferrite, and cobalt ferrite. A solid solution of the above ferrites canalso be deposited by these methods. In addition, films of metal oxides,for example, magnesium oxide, can also be deposited in singlecrystalline form by these methods.

The substrates useful for epitaxial growth of single crystal filmspreferably have lattice constants which approximate that of the ferriteto be deposited. Examples of generally useful substrates includemagnesium oxide, ferrites with spinel structures, such as manganese zincferrite, magnetite and magnesium ferrite, and sapphire.

Iclaim:

1. A vapor transport method for the deposition of single crystal ferritefilms comprises the steps of reacting at a temperature of from 850 C.-ll50 C. metal oxides selected from the group consisting of oxides of theconstituent metals of said ferrite and said ferrite with a reactivehalogen-containing gas in an open gas flow system to form gaseousreaction products consisting of metal halides and water vapor and theconverting said gaseous reaction products to said single crystal ferritefilm on a single crystal substrate positioned in said open gas flowsystem by causing a shift in equilibrium conditions of said gaseousreaction products in the vicinity of said substrate, the temperature ofsaid metal oxides being maintained during deposition at a difference offrom C. to 30 C. greater than the temperature of said substrate suchthat when the temperature difference is 0 C., said shift in equilibriumis caused by the addition of water vapor to the gaseous products andwhen said temperature difference is greater than 0 C. the saiddifference shall be between 20-30 C., said temperature differencecreating said shift in equilibrium and the rate of said 7 gas flow beingless that 10 ml. per minute with said substrate positioned from l-l 0mm. from said metal oxides.

2. A close-space vapor transport method for the formation of singlecrystal ferrite films comprising the steps, as recited in claim 1,wherein said reactive halogen-containing gas comprises a hydrogen halidegas selected from hydrogen chloride and hydrogen bromide, said reactivegas flowing at a rate of about 1-10 ml. per minute, and wherein saidshift in equilibrium occurs at the surface of said substrate by causingsaid substrate to be at a temperature of from -30 C. lower than that ofsaid heated metal oxides and said substrate being separated from saidmetal oxides by a distance of from l to 10 mm.

3. An isothermal transport method for the formation of single crystalferrite films comprising the steps as recited in claim '1 wherein saidreactive halogen-containing gas comprises a gas selected from hydrogenchloride and hydrogen bromide and wherein said shift in equilibrium iscaused by the addition of water vapor into said gaseous reactionproducts in the vicinity of said substrate.

4. The isothermal transport method recited in claim 3 wherein said metaloxides are comprised of powdered individual oxides of the constituentmetals in said ferrite film to be prepared and wherein said oxides areheated to a temperature between about 850 C. and ll50 C., said hydrogenhalide gas is mixed with an inert gas to form a gas mixture having aflow rate of at least several hundred mL/min.

5. The isothermal transport method recited in claim 3 wherein one ofsaid metal oxides is ferric oxide and wherein the gas mixture flowingover said ferric oxide includes oxygen.

6. The isothermal transport method recited in claim 3 wherein one ofsaid metal oxides is ferric oxide and the gas mixture flowing over saidferric oxide contains oxygen in a ratio of oxygen to hydrogen halide offrom about one-tenth to one-hundredth.

7. A close-space vapor transport method for the epitaxial deposition ofsingle crystal ferrite films on a single crystal substrate comprises thesteps of reacting a powdered ferrite having substantially the samecomposition as the ferrite film to be deposited with a hydrogen halidegas selected from hydrogen chloride and hydrogen bromide in an open gasflow system at a temperature between about 950 C. and 1 C., saidhydrogen halide gas being mixed with an inert gas to form a gas mixture,the flow rate of which is less than about 10 ml./min., less than 5ml./min. of which is said hydrogen halide gas, and wherein saidsubstrate is heated to a temperature of from about 20 to 30 lower thanthat of said powdered ferrite and said substrate being separated fromsaid powdered ferrite by a distance offrom about 1 to 5 mm.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pa 3,617,381 DatedNovember 2 1971 Inventor(s) Joseph John Hanak It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

In the specification, column 3, line 20, the formula that reads M 20(s)+bh8HX(g)=MX (g)+2Fe X (g)+4H 0(g) should read:

M Pe O (s) +8HX(g)=MX [g)+2FeX (g)+4H O (g) Column 5, line 24, "glasses"should read -glass--, column 6, line 25, "singly" should read single-.

Signed and sealed this 18th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents ORM PO-IOSO (10-69) USCOMM-DC eoavm 0 U 5 GOVERNMENT PRINYINGOFFILF IQBDO- 165434

2. A close-space vapor transport method for the formation of singlecrystal ferrite films comprising the steps, as recited in claim 1,wherein said reactive halogen-containing gas comprises a hydrogen halidegas selected from hydrogen chloride and hydrogen bromide, said reactivegas flowing at a rate of about 1-10 ml. per minute, and wherein saidshift in equilibrium occurs at the surface of said substrate by causingsaid substrate to be at a temperature of from 20*-30* C. lower than thatof said heated metal oxides and said substrate being separated from saidmetal oxides by a distance of from 1 to 10 mm.
 3. An isothermaltransport method for the formation of single crystal ferrite filmscomprising the steps as recited in claim 1 wherein said reactivehalogen-containing gas comprises a gas selected from hydrogen chlorideand hydrogen bromide and wherein said shift in equilibrium is caused bythe addition of water vapor into said gaseous reaction products in thevicinity of said substrate.
 4. The isothermal transport method recitedin claim 3 wherein said metal oxides are comprised of powderedindividual oxides of the constituent metals in said ferrite filM to beprepared and wherein said oxides are heated to a temperature betweenabout 850* C. and 1150* C., said hydrogen halide gas is mixed with aninert gas to form a gas mixture having a flow rate of at least severalhundred ml./min.
 5. The isothermal transport method recited in claim 3wherein one of said metal oxides is ferric oxide and wherein the gasmixture flowing over said ferric oxide includes oxygen.
 6. Theisothermal transport method recited in claim 3 wherein one of said metaloxides is ferric oxide and the gas mixture flowing over said ferricoxide contains oxygen in a ratio of oxygen to hydrogen halide of fromabout one-tenth to one-hundredth.
 7. A close-space vapor transportmethod for the epitaxial deposition of single crystal ferrite films on asingle crystal substrate comprises the steps of reacting a powderedferrite having substantially the same composition as the ferrite film tobe deposited with a hydrogen halide gas selected from hydrogen chlorideand hydrogen bromide in an open gas flow system at a temperature betweenabout 950* C. and 1100* C., said hydrogen halide gas being mixed with aninert gas to form a gas mixture, the flow rate of which is less thanabout 10 ml./min., less than 5 ml./min. of which is said hydrogen halidegas, and wherein said substrate is heated to a temperature of from about20* to 30* lower than that of said powdered ferrite and said substratebeing separated from said powdered ferrite by a distance of from about 1to 5 mm.